I don't think it will be Zoroark, because I believe Zorua will be a possible starter for marketing reasons. The Sandile line isn't strong enough to be the big bad. They may be cunning enough however.
Hydreigon/Sazandorah sounds like a likely candidate.
The big bad isn't just the final boss/villain. It's the man behind the scenes manipulating everything. The badass leader you fight once or twice is NOT the big bad. The guy who was using the guy you had frequent confrontational encounters with as his muscle is the big bad. For example N isn't the big bad, the honor goes to Geechisu/Dennis.